Megakaryocytes are the hematopoietic cells, largely found in the bone marrow, but also in peripheral blood and perhaps other tissues as well, which produce platelets (also known as thrombocytes) and subsequently release them into circulation. Megakaryocytes, like all of the hematopoietic cells of the human hematopoietic system, ultimately derive from a primitive stem cell after passing through a complex pathway comprising many cellular divisions and considerable differentiation and maturation.
The platelets derived from these megakaryocytic cells are critical for initiating blood clot formation at the site of injury. Platelets also release growth factors at the site of clot formation that speed the process of wound healing and may serve other functions. However, in patients suffering from depressed levels of platelets (thrombocytopenia) the inability to form clots is the most immediate and serious consequence, a potentially fatal complication of many therapies for cancer. Such cancer patients are generally treated for this problem with platelet transfusions. Other patients frequently requiring platelet transfusions are those undergoing bone marrow transplantation or patients with aplastic anemia.
Platelets for such procedures are obtained by plateletpheresis from normal donors. Like most human blood products, platelets for transfusion have a relatively short shelf-life and also expose the patients to considerable risk of exposure to dangerous viruses, such as the human immunodeficiency virus (HIV).
Clearly the ability to stimulate endogenous platelet formation in thrombocytopenic patients with a concomitant reduction in their dependence on platelet transfusion would be of great benefit. In addition the ability to correct or prevent thrombocytopenia in patients undergoing radiation therapy or chemotherapy for cancer would make such treatments safer and possibly permit increases in the intensity of the therapy thereby yielding greater anti-cancer effects.
For these reasons considerable research has been devoted to the identification and purification of factors involved in the regulation of megakaryocyte and platelet production. Although there is considerable controversy, the factors regulating the growth and differentiation of hematopoietic cells into mature megakaryocytes and the subsequent production of platelets by these cells are believed to fall into two classes: (1) megakaryocyte colony-stimulating factors (meg-CSFs) which support the proliferation and differentiation of megakaryocytic progenitors in culture, and (2) thrombopoietic (TPO) factors which support the differentiation and maturation of megakaryocytes resulting in the production and release of platelets. [See, e.g., E. Mazur, Exp. Hematol., 15:340-350 (1987).]
Either class of factors is defined by bioassay. Factors with meg-CSF activity support megakaryocyte colony formation, while factors with TPO activity elicit an elevation in the numbers of circulating platelets when administered to animals. It is not clear how many species of factors exist that have either or both of these activities. For example, human IL-3 supports human megakaryocyte colony formation and, at least in monkeys, also frequently elicits an elevation in platelet count. However, IL-3 influences hematopoietic cell development in all of the hematopoietic lineages and can be distinguished from specific regulators of megakaryocytopoiesis and platelet formation which interact selectively with-cells of the megakaryocytic lineage.
From the studies reported to date, it is not clear whether activities identified as meg-CSF also have TPO activity or vice versa. Many different reports in the literature describe factors which interact with cells of the megakaryocytic lineage. Several putative meg-CSF compositions have been derived from serum [See, e.g., R. Hoffman et al, J. Clin. Invest., 75:1174-1182 (1985); J. E. Straneva et al, Exp. Hematol., 15:657-663 (1987); E. Mazur et al, Exp. Hematol., 13:1164-1172 (1985]. A larger number of reports of a TPO factor are in the art. [See, e.g., T. P. McDonald, Exp. Hematol., 16:201-205 (1988); T. P. McDonald et al, Biochem. Med. Metab. Biol., 37:335-343 (1987); T. Tayrien et al, J. Biol. Chem., 262:3262-3268 (1987) and others].
Although there have been numerous additional reports tentatively identifying such regulatory factors, the biochemical and biological identification and characterization of these factors has been hampered by the small quantities of the naturally occurring factors available from natural sources, e.g., blood and urine. At present there is no identification of a single purified factor useful as a meg-CSF or TPO for pharmaceutical use in replacing serum-derived products or platelets.
There remains a need in the art for additional proteins purified from their natural sources or otherwise produced in homogeneous form, which are capable of stimulating or enhancing the production of platelets in vivo, to replace presently employed platelet transfusions.